This invention relates to energy-responsive systems, such as intruder detection systems, of the type which sense an occurrence of a condition, such as the presence of an intruder, in a region monitored by the system by sensing a predetermined change in the energy received by the energy-sensing component of the system. More particularly, this invention relates to improvements in energy-responsive systems of the type in which one or more energy-sensitive receivers transmits a condition-modulated electrical signal along a cable to a remote signal processing unit.
A so-called "multihead" intruder detection system comprises a plurality of intruder-sensing elements or receivers positioned at various locations in a relatively large space which is to be monitored for intrusion. Each receiver is sensitive to a particular type of energy (e.g., acoustic, infrared, microwave, etc.) which can be produced or modulated by the entrance of an intruder into the protected space, and each receiver is adapted to transmit electrical signals, proportional to the energy received, to a remote central processing unit which serves to process the electrical signals to determine whether the information contained thereby is indicative of the presence of an intruder. Typically, such signals are transmitted to the processing unit on a two-wire shielded cable known as a "shielded pair".
In installing multihead intruder detection systems, one often finds it useful to be able to monitor the system noise level as each receiver is connected to the system, inasmuch as the noise level is related to the number and placement of the receivers. Ideally, there should be a predetermined margin of safety between the system noise level and the threshold level at which an alarm is ultimately sounded to indicate the presence of an intruder. Because the signal processing unit is usually located some distance from most of the receivers, the installer must either rely on someone stationed at the processing unit to advise him when the desired margin of safety is reached or, alternatively, he must have a test cable which extends back to the control unit so that he can monitor, by a voltmeter or the like, the system noise level. Both of these approaches, of course, have certain obvious drawbacks. While this noise signal could be transmitted to each remote receiver by adding one or more additional wires to the cable which links the receivers with the central processing unit, this approach is undesirable from an economic standpoint.
Some commercially available multihead intruder detection systems have an indicating lamp in each receiver and comprise means for energizing such lamp when the particular receiver in which the lamp is mounted detects the presence of an intruder. It is known to transmit a lamp-energizing signal from the central processing unit to the receiver. While it is desirable for an installer to know when the system trips in response to the condition of interest, it also would be useful to have an indication when the system noise exceeds a predetermined margin of safety. No prior art system incorporates such a feature or, for that matter, the feature of being able to monitor the system noise at each remote receiver without the need for additional wiring beyond the shielded pair.